1. Technical Field
The present invention relates to solid polymer electrolyte fuel cells used for portable power sources, electric vehicle power sources, domestic cogeneration systems, etc.
2. Background Art
A fuel cell using a solid polymer electrolyte generates electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as the air. This fuel cell is basically composed of a polymer electrolyte membrane for selectively transporting hydrogen ions, and a pair of electrodes, namely, an anode and a cathode, formed on both surfaces of the polymer electrolyte membrane. The above-mentioned electrode usually includes a catalyst layer which is composed mainly of carbon particles carrying a platinum metal catalyst and formed on the surface of the polymer electrolyte membrane, and a diffusion layer which has both gas permeability and electronic conductivity and is formed on the outer surface of this catalyst layer.
Moreover, a gas sealing material or gaskets are arranged on the peripheral portions of the electrodes with the polymer electrolyte membrane therebetween so as to prevent a fuel gas and an oxidant gas supplied to the electrodes from leaking out or prevent two kinds of gases from mixing together. These sealing material and gaskets are assembled into a single part together with the electrodes and polymer electrolyte membrane in advance. This part is called the xe2x80x9cMEAxe2x80x9d (electrolyte membrane and electrode assembly). Disposed outside of the MEA are conductive separator plates for mechanically securing the MEA and for connecting adjacent MEAs electrically in series, or in parallel in some case. A portion of the separator plate, which is in contact with the MEA, is provided with a gas flow path for supplying a reacting gas to the electrode surface and for removing a generated gas and an excess gas. Although the gas flow path can be provided separately from the separator plate, grooves are usually formed on the surface of each separator to serve as the gas flow path.
In order to supply the fuel gas and oxidant gas to such grooves, it is necessary to branch pipes for supplying the fuel gas and oxidant gas, respectively, according to the number of separator plates to be used, and to use piping jigs for connecting an end of the branch directly to the grooves of the separator plate. This jig is called xe2x80x9cmanifoldxe2x80x9d, and a type of manifold which directly connects the supply pipes of the fuel gas and oxidant gas to the grooves as mentioned above is called the xe2x80x9cexternal manifoldxe2x80x9d. There is a type of manifold, called the xe2x80x9cinternal manifoldxe2x80x9d, with a more simple structure. The internal manifold is one in which through apertures are formed in the separator plates having a gas flow path and the inlet and outlet of the gas flow path are extended to the apertures so as to supply the fuel gas and oxidant gas directly from the apertures.
Since the fuel cell generates heat during operation, it is necessary to cool the cell with cooling water or the like so as to keep the cell in good temperature conditions. In general, a cooling section for feeding the cooling water is provided for every one to three cells. There are a type in which the cooling section is inserted between the separator plates and a type in which a cooling water flow path is provided on the rear surface of the separator plate so as to serve as the cooling section, and the latter is often used. The structure of a common cell stack is such that the MEAs, separator plates and cooling sections are placed one upon another to form a stack of 10 to 200 cells, and this cell stack is sandwiched by end plates, with a current collector plate and an insulating plate between the cell stack and each end plate, and secured with a clamping bolt from both sides.
In such a solid polymer electrolyte fuel cell, the separator plates need to have a high conductivity, high gas tightness with respect to a fuel gas and oxidant gas, and high corrosion resistance against a reaction of hydrogen/oxygen oxidation-reduction. For such reasons, conventional separator plates are usually formed from carbon materials such as glassy carbon and expanded graphite, and the gas flow path is formed by cutting the surface of the separator plate, or by molding with a mold when the material is expanded graphite.
In a conventional method including cutting a carbon plate, it is difficult to reduce the cost of the material of the carbon plate and the cost of cutting the carbon plate. Besides, a method using expanded graphite requires a high cost of material, and it has been considered that the high cost of material prevents a practical use of this method.
In resent years, there have been attempts to use a metal plate such as stainless steel in place of the conventionally used carbon material.
However, in the above-mentioned method using a metal plate, since the metal plate is exposed to an acidic atmosphere of the pH of 2 to 3 at high temperatures, the corrosion or dissolution of the metal plate will occur when used in a long time. The corrosion of the metal plate increases the electrical resistance in the corroded portion and decreases the output of the cell. Moreover, when the metal plate is dissolved, the dissolved metal ions diffuse into the polymer electrolyte membrane and are trapped by the ion exchange cite of the polymer electrolyte membrane, resulting in a lowering of the ionic conductivity of the polymer electrolyte membrane. For these causes, when a cell in which a metal plate is used as it is for a separator plate is operated for a long time, a problem arises that the power generating efficiency is gradually lowered.
An object of the present invention is to improve a separator plate for use in fuel cells and provide a separator plate which maintains chemical inactivity even when its surface to come in contact with a gas is exposed to an acidic atmosphere, suppresses corrosion and dissolution, and has good conductivity by using a metal that can readily be processed as a material.
The present invention provides a solid polymer electrolyte fuel cell comprising: a solid polymer electrolyte membrane; an anode and a cathode sandwiching the solid polymer electrolyte membrane therebetween; an anode-side conductive separator plate having a gas flow path for supplying a fuel gas to the anode; and a cathode-side conductive separator plate having a gas flow path for supplying an oxidant gas to the cathode, wherein each of the anode-side and cathode-side conductive separator plates is composed of a metal and a conductive coat which has resistance to oxidation and covers a surface of the metal.
The conductive coat is preferably selected from the group consisting of a carbonaceous coat, a conductive inorganic compound coat, and a metal-plated coat containing particles of a water repellent material.
A preferred conductive separator plate is composed of a spongy metal and a carbon powder layer which is filled into the spongy metal and covers the surface of the spongy metal.
Another preferred conductive separator plate is composed of a metal plate and a conductive coat covering the surface of the metal plate, wherein the conductive coat is a conductive inorganic compound selected from the group consisting of oxides, nitrides and carbides.
Still another preferred conductive separator plate is composed of a metal plate and a conductive coat covering the surface of the metal plate, wherein the conductive coat is made of a metal-plated coat containing particles of a water repellent material.
The present invention provides a solid polymer electrolyte fuel cell comprising anode-side and cathode-side conductive separator plates, each of which is formed by a metal whose surface is covered with a coat having resistance to oxidation, wherein at least surfaces of the separator plates which face an anode and cathode are roughened to have recessions and protrusions, and portions of the top surface of the protruding portions, which lack the coat, are electrically connected to the anode and cathode, respectively.
Moreover, the present invention provides a solid polymer electrolyte fuel cell comprising anode-side and cathode-side conductive separator plates, each of which is formed by a metal whose surface is covered with a coat having resistance to oxidation, wherein portions of the separator plates"" surface facing an anode and cathode, which lack the coat, are electrically connected to the anode and cathode, respectively, through conductive particles interposed between the separator plate and the anode and between the separator plate and the cathode.
The present invention provides a conductive separator plate composed of a metal plate having grooves or ribs for guiding a fuel gas or oxidant gas on its surface facing an electrode, and an insulating sheet which forms a gas flow path for guiding the fuel gas or oxidant gas from a gas supply side to a gas discharge side on a surface of the metal plate in cooperation with the grooves or ribs and has elasticity to function as a gasket for preventing the fuel gas or oxidant gas from leaking out of the gas flow path.